1. Field of the Invention
The invention relates to a rotation angle sensor that detects a rotation angle of a rotary shaft.
2. Discussion of Background
US2011/0087456 A1 describes a device as a conventional rotation angle sensor of the above-described type. The rotation angle sensor includes two bridge circuits and a bias magnet. Each of the two bridge circuits is formed of four magnetoresistance elements. The bias magnet applies a bias magnetic field to each of the magnetoresistance elements of the two bridge circuits. The bias magnet rotates together with the rotary shaft. The direction of the bias magnetic field that is applied to each magnetoresistance element changes as the bias magnet rotates, and the resistance value of each magnetoresistance element changes. Thus, one of the bridge circuits outputs a signal +sin θ that sinusoidally changes with respect to a rotation angle θ of the rotary shaft, and a signal +cos θ that sinusoidally changes with respect to the rotation angle θ. The other one of the bridge circuits outputs a signal −sin θ that is shifted in phase by 180° with respect to the signal +sin θ, and a signal −cos θ that is shifted in phase by 180° with respect to the signal +cos θ. Thus, if the detection signals output from the rotation angle sensor are acquired, and an arctangent is computed on the basis of the signal +sin θ and the signal +cos θ or an arctangent is computed on the basis of the signal −sin θ and the signal −cos θ, it is possible to detect the rotation angle θ of the rotary shaft.
With the above-described configuration, for example, even if the signal +cos θ from the one of the bridge circuits becomes abnormal, it is possible to compute the rotation angle θ by computing an arctangent on the basis of the signal −sin θ and the signal −cos θ. In addition, even if the signal −sin θ from the other one of the bridge circuits also becomes abnormal, it is possible to compute the rotation angle θ by reversing one of the sign of the remaining signal +sin θ and the sign of the remaining signal −cos θ and then computing an arctangent. Thus, it is possible to ensure the redundancy.
In the rotation angle sensor described in US2011/0087456 A1, if the signal +cos θ from the one of the bridge circuits becomes abnormal and then the signal −cos θ from the other one of the bridge circuits also becomes abnormal, normal signals are the signal +sin θ and the signal −sin θ. However, it is not possible to compute an arctangent on the basis of these signals. Consequently, it is not possible to detect the rotation angle θ of the rotary shaft. That is, in the rotation angle sensor described in US2011/0087456 A1, if two of the four detection signals become abnormal and the remaining normal signals are only the signals of sin θ or only the signals of cos θ, it is not possible to compute the rotation angle θ.